Rhococo Retro Home COlor COmputer
|Project Rhococo Retro Home COlor COmputer|
|Een nieuwe 8-bit homecomputer met moderne onderdelen|
Less is more, less complexity, more fun is the driving force behind the Rhococo Retro Home COlor Computer. It will be a remake home-computer inspired on, but not compatible with, the Sinclair spectrum, Initially I considered not basing it on a Z80, but on an ARM7TDMI based simple micro-controller, (probably an STR710FZ2) running at 66MHz with 256 + 16 KB internal fast Flash and with 64K internal fast RAM and a separate external 32K VRAM. But early 2017 I changed my mind, as you can read below.
The idea behind it is bringing back a new homecomputer as it was popular in the 80's, but build with modern components.
It will be capable of 320x200 VGA full high resolution graphics 16 colour display (Mode "D" timings, 70Hz, Horizontal Sync = 31.778 KHz, pixel clock 25.175 MHz).
To be able to keep the design simple, but still able to be drive a VGA monitor, I chose to support only the lowest VGA resolution of 320 x 200 with 16 colours. It means we only need to have, (and manipulate) a 256000 bit (32K) frame buffer. Three bytes will be shifted to the Red, Green an Blue colour inputs, and one to the intensity input, giving sixteen colours per pixel. Preferably the "glue logic" for it will consist of standard logic, but if necessary I will consider a small FPGA.
the Rhococo will support a simple SD-Card interface as storage, and will use a keyboard similar to the ZX81+34, but with more keys,and a more conventional keyboard layout.
For sound I consider a PWM output.
It will be a "turn on and use" system, meaning that the "OS", and a programming language will permanently reside in ROM, ready the moment you turn the Rhococo on (Meaning it will be based on some kind of interpretative language).
For the moment (quarter 4 2015) the Rhococo has been stalled, as I have problems getting the ZX81+34 ZX81 clone to work. Now working on version two.
April 18, 2017 Changed my mind, will try to build a Z80 based more "home computer like design", with a CRTC, and not software driven
After a year or so contemplating this idea of a new homecomputer, I came to the conclusion that an ARM based system with software VGA generation might be too complex for a the simple system I have in mind. I will make a NEW homecomputer design with components that can be bought in 2017 (and the next few decades). It will be based on a Z80 compatible CPU as it is one of the few 8-bit CPU's that is still relatively easy to get, and it was so popular in the 80's that there is a plethora of software available for them. One thing I don't want is to use an FPGA for it, as these are still relatively expensive and difficult to program for. This does cause a problem, as most old homecomputers either used some kind of programmable logic (Sinclairs ULA logic) or were based around a CRTC like the MC6845 which are no longer available, or they used a VDC (video display controller) like the TMS9918 (MSX an many others) which are also no longer available. My preference would have been using a CRTC, but as I said I can find no CRTC's for sale anymore, I could build one with standard logic (like the TRS-80 did) but that's too RETRO even for me, (uses far too much components). Instead I'm wondering if I can replace the CRTC with an simple AT-mega (Arduino) microcomputer. For a 320 x 200 display you would need 320 / 8 = 40 8-pixel wide rows and 200 lines, which means you need 40 * 200 = 8000 address locations, this is about 8191 or 2 ^ 13, meaning you need 13 address lines to address these 8191 (or hex 0x1FFF) locations. and a couple of other signals like HSYNC and VSYNC and video clock. An Arduino (nano) uses an ATmega328, and doesn't really have that many GPO (output) ports, but that can be solved with a few 8-bit latches. I'm planning to do this in little steps, so first I'm planning to connect 8K Byte SRAM (or an EPROM for testing) to an Arduino nano, plus a shift register to clock out a video stream, to try to generate a 320 x 200 (monochrome) VGA signal, using a Arduino nano as a CRTC surrogate. Then continue from there.